pctrie: correct iter node after node allocation

[dougm]

Summary

Usually, a pctrie_iter operation leaves the iterator node pointing to the last pctrie_node on the path to the found, or inserted, or removed leaf. However, that is not the case when inserting a leaf causes the allocation of a new pctrie node. In that case, the iterator node is left pointing to the parent of the allocated node, and not to the newly allocated node itself, the parent of the just-inserted leaf. Correct this anomaly.